The field of the invention generally relates to a method for bending tubes, and more particularly relates to bending tubes to form tubular heat exchangers for residential furnaces.
Recently, residential furnaces have been constructed using tubular heat exchangers instead of the more conventional clam-shell heat exchangers. With such arrangement, a plurality of stainless steel or aluminized steel tubes are provided, and one end of each is fired by an individual burner orifice. The combustion gases heat the tubes, and the heat is transferred to household return air that is passed across the tubes within a heat exchange chamber of the furnace. In one furnace embodiment, the combustion gases are then exhausted; in an alternate furnace embodiment, the combustion gases are then directed from the tubes to a recuperative heat exchanger so as to increase the efficiency of the furnace.
In the above-described furnace application, it is desirable to maximize the heat exchange surface area within the confined or restricted volume inside the heat exchange chamber. Accordingly, each tube is bent into a serpentine configuration so as to increase the length of each tube that will fit into the chamber. Typically, the tubes have a 1.75-inch outer diameter (OD) and a wall thickness (WT) of 0.035 inches. Each of the bends is 180.degree. and has a relatively tight centerline radius (CLR) such as, for example, 2.5 inches. The bends are made using a conventional rotary bend die with a linked-ball mandrel. More specifically, a tube is seated in the groove of the rotary bend die that has a wiper die positioned adjacent thereto. Conventionally, the wiper die has a corresponding tangential groove with a knife edge that conforms to the bend die groove so as to prevent wrinkling of the tube at the tangent point. Next, a pressure die and clamp die are moved up against the opposite side of the tube with the pressure die pressing the pipe against the wiper die and the clamp die clamping a front portion of the tube to the bend die. The bend die and clamp die are then rotated approximately 180.degree. while the pressure die moves forward linearly carrying the tube tangentially to the bend point. In conventional manner, a ball mandrel is positioned inside the tube during the bending process, and it advances with the tube around the bend so as to prevent the tube from collapsing. Next, the ball mandrel, the pressure die and the clamp die are retracted, and the tube is removed from the bend die by applying a relatively small removal force. In one furnace configuration, each tube is bent in three locations thus providing four parallel segments. In an alternate configuration, each tube is bent in five locations thus providing six parallel segments. Each tube is also rotated on its axis in altering directions after each bend so as to limit the vertical height of the tubular heat exchanger; this also provides for more dense packing of the segments of the tube within the heat exchange chamber.
The above-described method of bending tubes or pipes has a number of disadvantages. First, the wiper dies and the ball mandrels wear out or break at a relatively fast rate and are expensive to replace. Second, lubrication is conventionally applied so as to reduce the wear on the ball mandrels and on the knive edge of the wiper die. After the tubes have been bent, the lubrication has to be cleaned from the tubular heat exchangers, and this involves additional labor. Further, there are problems and costs associated with disposing of the used lubrication. Third, the rejection rate--i.e. the percentage of tubular heat exchangers that fail to pass inspection--is relatively high with the above-described method of bending. One factor that contributes to the high rejection rate is that the above-described internal multi-ball mandrel bending technique may cause excessive thinning of the outer wall of the tube. More specifically, such technique normally causes the neutral axis--the transition point between compression on the inside of the bend and tension on the outside of the bend--to be located toward the inside of the bend or typically about a third of the way from inside to outside. As a result, a tube with a wall thickness of 0.035 inches may typically be thinned to approximately 0.028 inches on the outside, and this puts relatively high stress on the tubing and particularly its weld seam. Another factor that contributes to the high rejection rate is that as the multi-ball mandrel is extracted from the bent tube, it wears against the ridges on the inside of the bend and smoothes them down or bends them over.
For some industry applications, tubes have been bent without the use of a mandrel. Also, controlled-wrinkle compression bend dies have been used. However, bending without the use of a mandrel is generally reserved for bends that are less than 180.degree. and with tubing that has relatively thick walls. More specifically, as a general rule, it is thought that the Bending Factor of such bends should not exceed 12, and generally should be in the range 4-7. Here, Bending Factor is defined as EQU Bending Factor=Wall Factor.div.(CLR.div.OD)
where Wall Factor is the outer diameter of the tube divided by the wall thickness, CLR is the centerline radius of the bend, and OD is the outer diameter of the tube. However, 12 is much too low a Bending Factor for the tube and bending parameters which are most advantageous for a residential furnace application. For example, to attain a Bending Factor of 12 for a 2.5-inch CLR bend using 1.75-inch OD tube, the wall thickness would have to be increased to approximately 0.1 inches, but this tube would not be cost effective to use. Alternatively, to attain a bending factor of 12 using a 1.75-inch OD tube with a wall thickness of 0.035 inches, the centerline radius would have to be increased to approximately 7.3 inches; this bend, however, would not be tight enough to optimize the heat exchange surface area within the heat exchange chamber.